Interspire seems to actively be updating Email Marketer. They released version 6.4.1 in October. That version includes a few improvements related to authentication. From the release notes:
Released: October 22nd, 2021
- Added two factor authentication feature
- Added admin can force users to select and configure their own SMTP server
- Added self signed certificate setting option
- Process UTF-8 encoded subject lines in bounced messages
- Fixed export statistics count issue